BACKGROUND: Childhood obesity is an increasingly prevalent health problem affecting 22-27% of the children and adolescents in the United States (NHANES II). While epidemiological studies have provided indicators as to which children are at risk of becoming obese, we still do not know why they become obese. The literature suggests that alterations in energy expenditure may play a role in the etiology of obesity. HYPOTHESIS: Reduced energy expenditure and nutrient partitioning favoring fat deposition result in higher fat gain in children predisposed to obesity. OBJECTIVES: To test if the following putative factors lead to higher fat gain in children predisposed to obesity: 1) lower maintenance energy expenditure, 2) lower levels of habitual physical activity, 3) lower exercise economy and oxidative capacity in skeletal muscle, 4) lower rates of fat oxidation, and a higher proportion of fat to fat- free mass deposition. DESIGN: This prospective study will investigate antecedents to obesity in normal-weight, prepubescent girls (8 y) with either normal-weight (N=36) or obese (N=36) parents. Expected predictors of fat gain will be measured at baseline, and at 1 and 2 years later. Basal metabolic rate (BMR), sleeping metabolic rate, 24-h sedentary energy expenditure quid substrate utilization will be measured by room respiration calorimetry. Total energy expenditure (TEE) will be measured by doubly-labeled water and activity quantified as TEE/BMR. Pattern o habitual physical activity will be described by heart rate monitoring. VO2peak and VO2submaximal will be determined by treadmill/ergometer exercise testing, and in vivo muscle oxidative capacity by nuclear magnetic resonance spectroscopy. Body composition will be assessed by dual-energy x-ray absorptiometry and total body potassium at baseline, and at l and 2 years later to determine the outcome variable, fat gain. SIGNIFICANCE: Recently released figures from the NHANES III survey revealed a 40% increase in the prevalence of obesity among adolescents. Similar results are anticipated for children. The proposed prospective study will examine potential determinants of higher fat gain in children "at risk" of becoming obese. This study is important since significant health problems are associated with childhood obesity; identification of the etiological factors of childhood obesity is required to prevent and to reduce the prevalence of obesity.